The phrase "dubious assumption" means an assumption or belief that is likely to be untrue or unreliable. It suggests that the idea or belief might be questionable or doubtful. Full definition
Let's forget for a moment that that area doesn't really exist as anything other than a mathematical construct based on dubious assumptions. (judithcurry.com)
Under this highly dubious assumption, climate policy only shifts capital around between industries and provinces but can not affect total investment. (progressive-economics.ca)
I suspect that there are very dubious assumptions that reading the real report carefully will quickly disclose. (gigaom.com)